1. Field of the Invention
This invention relates to the formation of diamond films on a carbon-coated substrate. More particularly, this invention relates to a process for pretreating a carbon-coated substrate on which the diamond film is to be formed to thereby provide a smooth surface continuous diamond film over the underlying carbon-coated substrate.
2. Description of the Related Art
The development of a process for growth of high quality diamond films on various substrates using a plasma enhanced chemical vapor deposition technique has resulted in the exploration of various applications for use of the resulting outstanding physical properties of this unique material, including use as a hard protective coating, which employs diamond's very high hardness and low friction coefficient in air; as electronic (semiconductor) material, which employs its high resistance and hole mobility; as optical window material, which employs its transparency to visible, infrared, and most ultraviolet light; and as heat sink material, which employs its high thermal conductivity.
However, for all of these applications, a smooth and uniform film is needed because a rough diamond surface can cause mechanical abrasion of adjacent components; large surface light scattering, resulting in low transmission as an optical window; and poor physical contact, resulting in low heat transfer.
Since diamond films prepared by conventional chemical vapor deposition usually have a rough surface, the desired smooth diamond film is conventionally produced by polishing the diamond film after deposition. Such polishing is an effective tool for a thick (and flat) film used as a wear-resistant coating. However, such polishing techniques can cause surface damage and contamination of the film, and are not easily applied to thin films, or to films on substrates of complex geometric shape.
A superior approach to the surface roughness problem has been found to be control of the grain size or nucleation density during the diamond growth phase, since in principle, if the particle size of nucleation seeds can be reduced, and if a high and uniform density of the nucleation sites is maintained, and subsequent growth is uniform on all nuclei, then the surface roughness of the final film should be reduced accordingly.
Thus the problem of decreasing the surface roughness becomes a problem of increasing the diamond nucleation density. The most common technique which has been employed for obtaining high nucleation density of diamond grown on foreign substrates has been to mechanically abrade (scratch) the substrate with diamond powder or paste or other hard abrasive before deposition. However, such mechanical abrasion of the substrate is not compatible with many of the intended uses of the coating material, e.g., for use on microelectronic substrates.
While such mechanical abrasion of the substrate with diamond powder is not desirable for many applications, studies based on the use of this technique have indicated that such substrates scratched with diamond powder result in high diamond nucleation density. This has, in turn, given rise to a less destructive method of increasing the nucleation sites on a substrate, which comprises coating the substrate with carbon particles. However, it has been found that while the presence of carbon particles on the surface of a substrate may, under some circumstances, provide a high density of nucleation sites for subsequent diamond growth, the increased nucleation density is not always accompanied by film uniformity, thus giving rise to non-uniform diamond growth on the substrate.
More recently Meilunas et al., in a paper entitled "Nucleation of Diamond Films on Surfaces Using Carbon Clusters", published on Dec. 23, 1991 by the American Institute of Physics on pages 3461-3463, described a pretreatment of such a carbon (C.sub.60 /C.sub.70)-based nucleation layer, which comprises applying a negative voltage bias to the substrate of about 200 to 300 volt, while exposing the substrate to a plasma containing a higher concentration of methane (about 10%) in the methane/hydrogen gas mixture than normally used in the standard diamond deposition process, as well as using a lower pressure (about 15 Torr) during this pretreatment than normally used in the standard diamond deposition process.
While the results reported by Meilunas et al. indicate that such a pretreatment does result in the formation of a high density of nucleation sites for diamond growth, resulting in the formation of continuous diamond films, it would be desirable to provide a simpler pretreatment process in which it would be unnecessary to apply a bias voltage to the substrate during the pretreatment, and more importantly, to use other common carbons rather than exotic carbons such as C.sub.60 /C.sub.70 used by Meilunas et al.